1. Field of the Invention
The present invention relates to an optical head assembly for recording and reproducing data on and from an optical disk medium and to an optical information recording/reproducing device. In particular, the present invention relates to an optical head assembly and an optical information recording/reproducing device with which tangential tilt of optical recording media can be detected.
2. Description of the Prior Art
Recording density obtained in optical information recording/reproducing devices is in inverse proportion to the square of the diameter of focused light spots formed on an optical recording medium with an optical head assembly. Thus, the smaller the diameter of the focused light spot is, the higher the recording density is. The diameter of the focused light spot is in inverse proportion to the numerical aperture of the objective lens in the optical head assembly. Thus, the higher the numerical aperture of the objective lens is, the smaller the diameter of the focused light spot is.
Tilting of the optical recording medium in a tangential direction relative to the objective lens deforms the focused light spots due to coma aberration for which the substrate of the optical recording medium is responsible, badly affecting recording/reproducing properties. The coma aberration is in proportion to the cubic of the numerical aperture of the objective lens. Thus, the higher the numerical aperture of the objective lens is, the smaller a margin for the tangential tilt of the optical recording medium with respect to the recording/reproducing properties is. Accordingly, it is necessary to detect and correct the tangential tilt of optical recording media to avoid degradation of recording/reproducing properties in optical head assemblies and optical information recording/reproducing device that employ an objective lens with a higher numerical aperture for a higher recording density.
FIG. 17 shows a configuration of a conventional optical head assembly with which the tangential tilt of optical recording media can be detected. This optical head assembly is disclosed in Japanese Patent Laid-Open No. 9-161293. The light beam emitted from a semiconductor laser 105 is converted into parallel rays with a collimator lens 106. The parallel rays are then incident on a diffraction grating 107 where they are divided into the zero order diffracted beam, the plus first order diffracted beam and the minus first order diffracted beam.
These beams are directed to a half mirror 108 where about 50% of the light is passed through it and are focused on a disk D by an objective lens 109. The three beams reflected from the disk D are transmitted through the objective lens 109 in the opposite direction, about 50% of which are reflected from the half mirror 108. The reflected light is passed through a cylindrical lens 111 and a lens 112, and is then received by a photodetector 113. The photodetector 113 is disposed on the midway of the line focuses of the cylindrical lens 111 and the lens 112.
FIG. 18 is a plan view of the diffraction grating 107. The diffraction grating 107 serves to impart, to the plus and minus first order diffracted beams, a coma aberration in the tangential direction of the disk D. The direction of grating in the diffraction grating 107 is generally in parallel with the radial direction of the disk D. A grating pattern is such that the upper lines (upper half in the figure) are curved upward while the lower lines (lower half in the figure) are curved downward.
FIG. 19 shows location of the focused light spots on the disk D. Focused light spots L115, L116, and L117 correspond to the zero order diffracted beam, the plus first order diffracted beam and the minus first order diffracted beam, respectively, from the diffraction grating 107. These spots are located on the same track D1 where pits are formed. The focused light spots L116 and L117 have side lobes on upper and lower sides thereof, respectively, in the tangential direction of the disk D.
FIG. 20 shows a pattern of light receiving elements of the photodetector 113 and location of the focused light spots on the photodetector 113. A light spot L124 corresponds to the zero order diffracted beam from the diffraction grating 107 and is received by light receiving elements 118 to 121 which are four divisions defined by the dividing line parallel to the tangential direction of the disk D traversing across the optical axis and the other dividing line parallel to the radial direction. A light spot L125 corresponds to the plus first order diffracted beam from the diffraction grating 107 and is received by a single light receiving element 122. A light spot L126 corresponds to the minus first order diffracted beam from the diffraction grating 107 and is received by a single light receiving element 123. The row of the focused light spots L115 to L117 on the disk D is oriented in the tangential direction while the row of the light spots L124 to L126 on the photodetector 113 is oriented in the radial direction due to an effect of the cylindrical lens 111 and the lens 112 (the up-and-down direction corresponds to the radial direction and the side-to-side direction corresponds to the tangential direction in FIG. 20).
The outputs from the light receiving elements 118 to 123 are herein represented by V118 to V123, respectively. A focus error signal may be obtained, using the astigmatic method, from the following arithmetic operation:
(V118+V121)xe2x88x92(V119+V120). 
A tracking error signal may be obtained, using the push-pull method, from the following arithmetic operation:
(V118+V120)xe2x88x92(V119+V121). 
A playback signal produced by the focused light spot L115 may be obtained from the following arithmetic operation:
V118+V119+V120+V121. 
The tangential tilt of the disk D may be detected by either one of the following two approaches. The first approach is to obtain a tangential tilt signal by subtracting V123 from V122. The second approach is to obtain a tangential tilt signal according to a difference in bit error rates of a playback signal produced by the focused light spot L116 from the output V122 and a playback signal produced by the focused light spot L117 from the output V123.
When the first approach is used to detect the tangential tilt of an optical recording medium in the conventional optical head assembly, there is a defect of not being capable of detecting the tangential tilt at a high sensitivity because the change in the outputs V122 and V123 for the tangential tilt is significantly small.
On the other hand, when the second approach is used to detect the tangential tilt of the optical recording medium in the conventional optical head assembly, it is necessary to measure the bit error rates in the playback signals. The tangential tilt can be detected only with optical recording media for playback only, where such signals are recorded previously. No tangential tilt can be detected with recordable and rewritable optical recording media where no such signal is recorded previously.
Object of the Invention
An object of the present invention is to overcome the above-mentioned problem in the conventional optical head assembly with which the tangential tilt of the optical recording media can be detected. Another object of the present invention is to provide an optical head assembly and an optical information recording/reproducing device which allows detection of the tangential tilt at a high sensitivity and with which the tangential tilt can be detected even on the recordable and rewritable optical recording media where no signal is recorded previously.
An optical head assembly according to the present invention comprises: a light source; an objective lens which focuses transmitted light from the light source onto an optical recording medium; and a photodetector adapted to receive reflected light from the optical recording medium. The photodetector has light receiving elements each of which individually receives light components of the reflected light directed to regions on one side in the tangential direction of the optical recording medium, the regions being on both sides in the radial direction of the optical recording medium; light components of the reflected light directed to regions on the other side in the tangential direction of the optical recording medium, the regions being intermediate in the radial direction of the optical recording medium; light components of the reflected light directed to regions on one side in the tangential direction of the optical recording medium, the regions being intermediate in the radial direction of the optical recording medium; and light components of the reflected light directed to regions on the other side in the tangential direction of the optical recording medium, the regions being on both sides in the radial direction of the optical recording medium.
With the above-mentioned configuration, the reflected light from the optical recording medium is separated into the light components directed to the regions on one side in the tangential direction of the optical recording medium and on both sides in the radial direction of the optical recording medium; the light components directed to the regions on one side in the tangential direction of the optical recording medium and intermediate in the radial direction of the optical recording medium; the light components directed to the regions on the other side in the tangential direction of the optical recording medium and on both sides in the radial direction of the optical recording medium; and the light components directed to the regions on the other side in the tangential direction of the optical recording medium and intermediate in the radial direction of the optical recording medium. This allows detection of a tangential tilt (a tilt in the tangential direction, i.e., an angular change in the direction of rotation about the axis along the radial direction) of the optical recording medium according to changes in light intensity in these elements.
In other words, when there is a tangential tilt of the optical recording medium, the intensity is varied for, depending on whether the focused light spots are located on grooves or on lands (the valleys and tops of the grooves formed in the optical recording medium), the light components of the reflected light from the optical recording medium directed to the regions on one side in the tangential direction of the optical recording medium and on both sides in the radial direction of the optical recording medium; the light components thereof directed to the regions on one side in the tangential direction of the optical recording medium and intermediate in the radial direction of the optical recording medium; the light components thereof directed to the regions on the other side in the tangential direction of the optical recording medium and on both sides in the radial direction of the optical recording medium; and the light components thereof directed to the regions on the other side in the tangential direction of the optical recording medium and intermediate in the radial direction of the optical recording medium.
More specifically, when there is a positive tangential tilt of the optical recording medium and when the focused light spots are located on the grooves in the optical recording medium, or when there is a negative tangential tilt of the optical recording medium and when the focused light spots are located on the lands on the optical recording medium, the following applies.
The intensity is lower for the regions on one side in the tangential direction of the optical recording medium and on both sides in the radial direction of the optical recording medium as well as for the regions on the other side in the tangential direction of the optical recording medium and intermediate in the radial direction of the optical recording medium, than that obtained when there is no tangential tilt of the optical recording medium.
At the same time, the intensity is higher for the regions on one side in the tangential direction of the optical recording medium and intermediate in the radial direction of the optical recording medium as well as for the regions on the other side in the tangential direction of the optical recording medium and on both sides in the radial direction of the optical recording medium, than that obtained when there is no tangential tilt of the optical recording medium.
When there is a negative tangential tilt of the optical recording medium and when the focused light spots are located on the grooves in the optical recording medium, or when there is a positive tangential tilt of the optical recording medium and when the focused light spots are located on the lands on the optical recording medium, the following applies.
The intensity is higher for the regions on one side in the tangential direction of the optical recording medium and on both sides in the radial direction of the optical recording medium as well as for the regions on the other side in the tangential direction of the optical recording medium and intermediate in the radial direction of the optical recording medium, than that obtained when there is no tangential tilt of the optical recording medium.
At the same time, the intensity is lower for the regions on one side in the tangential direction of the optical recording medium and intermediate in the radial direction of the optical recording medium as well as for the regions on the other side in the tangential direction of the optical recording medium and on both sides in the radial direction of the optical recording medium, than that obtained when there is no tangential tilt of the optical recording medium.
Therefore, the tangential tilt of the optical recording medium can be detected according to the change in intensity of the light components reflected from the optical recording medium.
In the optical information recording/reproducing device according to the present invention, the above-mentioned optical head assembly is used. The driving circuit drives the tilt correction unit in response to a tangential tilt signal generated from the output of the photodetector of the optical head assembly. The tangential tilt of the optical recording medium is corrected so that any adverse effects are eliminated on recording/reproduction characteristics.
As apparent from the above, when the tangential tilt of the optical recording medium is detected in the optical head assembly and the optical information recording/reproducing device according to the present invention, the detection of the tangential tilt can be made at a high sensitivity because the intensity is varied significantly with the tangential tilt, in the light components reflected from the optical recording medium.
In addition, when the tangential tilt of the optical recording medium is detected in the optical head assembly and the optical information recording/reproducing device according to the present invention, the tangential tilt can be detected even with recordable and rewritable optical recording media where no signal is recorded previously, because the tangential tilt of the optical recording medium is detected according to the change in intensity of the light components reflected from the optical recording medium.